Temperature is the driving force for all biological activity. Consequently, the growth, development and reproduction of many organisms are predictable based on temperature (TBASE). The base temperatures are determined experimentally and are different for each organism. Growing degree-days (GDDs), while not perfect, are a more reliable method of predicting crop and insect development than calendar days. An important aspect of GDD is that no units (e.g. grams, litres or hectares) are associated with the value. Instead, the accumulated GDD values can be correlated with an event in an organism’s life.


There are two methods for calculating GDD. The first method is simpler and the second method involves higher level mathematics, but is more accurate, especially at cooler temperatures.

Method 1: Temperature averaging

To calculate GDDs, you must first find the mean temperature for the day. The mean temperature is found by adding together the high and low temperature for the day and dividing by two. If the mean temperature is at or below TBASE, then the Growing Degree Day value is zero. If the mean temperature is above TBASE, then the Growing Degree Day amount equals the mean temperature minus TBASE.

There are a couple things to remember when using the temperature averaging method. Since plants don’t grow any more at temperatures greater than 86 F than they do at temperatures less than 86 F, we use 86 F as the maximum temperature for any temperature greater than 86 F. Also, negative values are recorded as zero

Method 2: Baskerville-Emin method

The Baskerville-Emin method fits a curve to the various temperature points that are greater than the base temperature, then calculates GDDs from the area under that curve. That’s a little more math than most people can do on a piece of note paper, but it does a better job of calculating heat accumulation, particularly during the beginning of the growing season when temperatures are still cool. When temperatures are still cool in spring, referencing tools such as Michigan State University Enviroweather and finding nearby weather stations can help get more accurate GDD totals.

An example of the curve used to calculate GDDs using the Baskerville-Emin method


For the end it is important to know that, an understanding of plant and pest development at various GDDs can be helpful in making a variety of management decisions, like cutting alfalfa and corn silage, as well as decisions on scouting for insect pests.

Modified Growing Degree Days

Modified Growing Degree Days are like Growing Degree Days with several temperature adjustments. If the high temperature is above 86° F, it is reset to 86° F. If the low is below 50° F, it is reset to 50° F. Once the high / low temperatures have been modified (if needed), the average temperature for the day is computed and compared with a base temperature, which is usually 50° F. Modified Growing Degree Days are typically used to monitor the development of corn, the assumption being that development is limited once the temperature exceeds 86° F or falls below 50° F. For example, if the high for the day was 92° F and the low 68° F, the average for use in the modified GDD calculation would be 86 + 68 = 154 / 2 = 77


Tillage is defined as the mechanical manipulation of the soil for the purpose of crop production affecting significantly the soil characteristics such as soil water conservation, soil temperature, infiltration and evapotranspiration processes. This suggests that tillage exerts impact on the soil purposely to produce crop and consequently affects the environment .Tillage has been and will always be integral to crop production. Tillage can result in the degradation of soil, water, and air quality. Of all farm management practices, tillage may have the greatest impact on the environment. A wide variety of tillage equipment, practices and systems are available to farmers, providing opportunities to enhance environmental performance. Tillage also affects a variety of biophysical processes that impact the environment. These processes include: wind, water and tillage erosion, leaching and runoff, pesticide sorption and degradation, greenhouse gas emissions and soil carbon sequestration. Consequently, tillage has direct and indirect impacts on water, soil and air quality. The goal is to control soil properties such as water retention, temperature, infiltration, and evapotranspiration. Agriculture will most certainly abandon tillage practices in the future, relying on new technology to preserve the benefits of tillage while eliminating the related carbon emissions and soil degradation.

Types of Tillage

Tillage operations are broadly grouped into two types based on the time

  • On-season Tillage: It refers to tillage operations performed for crop production during the same season or at the start of the crop season.
  • Off-season TillageIt refers to tillage operations performed to prepare the soil for the upcoming main season crop. Off-season tillage may include:
    1. Post harvest tillage
    2. Summer tillage
    3. Winter tillage
    4. Fallow tillage


Harvest is the process of gathering mature crops from the fields. The goal of good harvesting is to maximize crop yield and minimize any crop losses and quality deterioration. Harvesting can be done manually, using hands or knifes and it can be done mechanically with the use of rippers, combine harvesters or other machines. Regardless of the method farmers use, several guidelines should be followed to ensure that harvest losses are minimum and crop quality is perserved during harvest operations, such as harvest time, method, duration, and postharvest processes. There are different methods farmers can use to determine the right time for harvesting: moisture content of grains, sugar and nutrient content of fruits, visual properties of mature fruits (color, scent, size), counting of the vegetation season days caracteristic for each variety, etc.

Principles of Harvesting

There are a few principles followed by farmers while harvesting:

  1. Slicing with a Sharp Smooth Edge
  2. Tearing with a Rough Serrated Edge
  3. Single Element with High-Velocity Impact and Sharp or Dull Edges
  4. Two Element Scissor Type or Shearing Type cutting


Evapotranspiration = Evaporation + Transpiration

Evapotranspiration is considered as one of the most important components of the hydrological cycle. On the Earth’s surface, evapotranspiration plays an important role in context of water-energy balance and irrigation, as well as agriculture practices. Efficient use of water resources in semiarid and arid agroecosystems of the World has become increasingly important because of rapid depletion of water resources, industrial development and population increase, drought conditions, and degradation of ground and surface water quality in many regions. As mentioned, evapotranspiration (ET), which is the sum of transpiration through plant canopy and evaporation from soil, plant, and open water surface, is the largest component of the hydrologic cycle. Furthermore, evapotranspiration includes water evaporation into the atmosphere from the soil surface, evaporation from the capillary fringe of the groundwater table, and evaporation from water bodies on land. Evapotranspiration also includes transpiration, which is the water movement from the soil to the atmosphere via plants. Transpiration occurs when plants take up liquid water from the soil and release water vapor into the air from their leaves.


Transpiration is the evaporation of water from plants. Most of the water absorbed by the roots of a plant—as much as 99.5 percent—is not used for growth or metabolism. it is excess water, and it leaves the plant through transpiration. Transpiration is very important for maintaining moisture conditions in the environment. As much as 10 percent of the moisture in the Earth’s atmosphere is from transpiration of water by plants.  Also, transpiration occurs through the stomatal apertures,and can be thought of as a necessary “cost” associated with the opening of the stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration cools plants, changes osmotic pressure of cells, and enables mass flow of mineral nutrients and water from roots to shoots.

Factors That Affect Transpiration

There are many factors that affect transpiration. One such factor is temperature. When temperatures increase, the stomata of leaves open and more water transpires. Plants that grow in warmer climates transpire more. Moisture levels of the air and soil are other important factors. When relative humidity of the air increases, there is more moisture in the air, so transpiration decreases. However, if there is more moisture in the soil, plants will transpire more because they are taking in more water. More wind also increases the rate of transpiration because it decreases the relative humidity around a plant. Of course, some plants also just transpire more than others. Plants that live in dry environments, such as cacti, have evolved to conserve water in part by transpiring less water. This allows them to thrive in arid regions like the desert. Also,  the size of the leaf because a leaf with a bigger surface area will transpire faster than a leaf with a smaller surface area.


Evaporation happens when a liquid is heated. This means that the liquid’s molecules have to gain kinetic energy. As a liquid’s molecules gain kinetic energy, the molecules begin to spread apart and vibrate faster. This causes the liquid to change its state of matter from liquid to gas. Water is a common substance where evaporation will take place. When energy or heat is added to water, the bonds that hold the molecule together begin to break, causing it to turn from a liquid into a gas. The temperature at which water will turn from a liquid to a gas is its boiling point: 212 degrees Fahrenheit or 100 degrees Celsius. It is also one of the three main steps in the global water cycle. Evaporation happens on a global scale. Also, accounts for 90 percent of the moisture in the Earth’s atmosphere, other 10 percent is due to plant transpiration. In the water cycle, evaporation occurs when sunlight warms the surface of the water. The heat from the sun makes the water molecules move faster and faster, until they move so fast they escape as a gas. Once evaporated, a molecule of water vapor spends about ten days in the air. As water vapor rises higher in the atmosphere, it begins to cool back down. When it is cool enough, the water vapor condenses and returns to liquid water. These water droplets eventually gather to form clouds and precipitation.


irrigation (also referred to as watering) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has been developed by many cultures around the world. The application of water to soil is essential for plant growth and it serves the  following functions:

  • It supplies moisture to the soil essential for the germination of seeds, and chemical and bacterial processes during plant growth
  • It cools the soil and the surroundings thus making the environment more favourable for plant growth.
  • It washes out or dilutes salts in the soil
  • It softens clods and thus helps in tillage operations.
  • It enables application of fertilisers.
  • It reduces the adverse effects of frost on crops.
  • It ensures crop success against short-duration droughts.

In several parts of the world, the moisture available in the root-zone soil, either from rain or from underground waters, may not be sufficient for the requirements of the plant life. This deficiency may be either for the entire crop season or for only part of the crop season. For optimum plant growth, therefore, it becomes necessary to make up the deficiency by adding water to the root-zone soil. This artificial application of water to land for supplementing the naturally available moisture in the root-zone soil for the purpose of agricultural production is termed irrigation. Irrigation water delivered into the soil is always more than the requirement of the crop for building plant tissues, evaporation, and transpiration. In some cases the soil may be naturally saturated with water or has more water than is required for healthy growth of the plant. This excess water is as harmful to the growth of the plant as lack of water during critical stages of the plant life. This excess water can be naturally disposed of only if the natural drainage facilities exist in or around the irrigated area.

Classification of Irrigation Systems

The classification of the irrigation systems is mainly based on the way the water is applied to the agricultural land. The main classifications of the systems are:

  • Gravity Irrigation System
  • Lift irrigation System
  • Combined System
  • Sprinkler Irrigation System

Environmental impacts

The environmental effects of irrigation relate to the changes in quantity and quality of soil and water as a result of irrigation and the subsequent effects on natural and social conditions in river basins. Negative impacts frequently accompany extensive irrigation. Some projects which diverted surface water for irrigation dried up the water sources, which led to a more extreme regional climate. Projects that relied on groundwater and pumped too much from underground aquifers created subsidence and salinization. Salinization of irrigation water in turn damaged the crops and seeped into drinking water. Pests and pathogens also thrived in the irrigation canals or ponds full of still water, which created regional outbreaks of diseases like malaria and schistosomiasis. Overdrafting (depletion) of underground aquifers: In the mid-20th century, the advent of diesel and electric motors led to systems that could pump groundwater out of major aquifers faster than drainage basins could refill them. This can lead to permanent loss of aquifer capacity, decreased water quality, ground subsidence, and other problems.


Subirrigation has been used for many years in field crops in areas with high water tables. It is a method of artificially raising the water table to allow the soil to be moistened from below the plants’ root zone. Often those systems are located on permanent grasslands in lowlands or river valleys and combined with drainage infrastructure. A system of pumping stations, canals, weirs and gates allows it to increase or decrease the water level in a network of ditches and thereby control the water table. Subirrigation can conserve nutrients and water, reduce labor costs, and helpgrowers meet environmental regulations.


Fertilizers are compounds or mixtures delivered as solids, liquids or gases, that supply essential nutrients to crops in soluble forms that are convenient and safe to handle. Fertilizers may be applied to the soil or directly to foliage. Generally, the farmer tries to obtain satisfactory yield at minimum cost in money and labour. Fertilizer application is believed to have been responsible for at least 50% increase in crop yield in the 20th century. Due to the growing population and consequent pressure of use, agricultural soils must maintain adequate levels of quantity and quality to produce food, fiber, and energy, without falling victim to a negative impact on their balance of nutrients, health, or their ability to function. The use of mineral fertilizers has long been a key tool to offset nutrient outputs and thus achieve increased yields.  Improper fertilizing technology might have a negative effect on soil health and soil-related ecosystem services. Imbalanced use of chemical fertilizers can alter soil pH, and increase pests attack, acidification, and soil crust, which results in a decrease in soil organic carbon and useful organisms, stunting plant growth and yield, and even leading to the emission of greenhouse gases.


Environmental impact

Carbon dioxide equivalent or CO2 equivalent (CO2eq) is a metric used to compare emissions from various greenhouse gases based on their global warming potential (GWP). The comparison is achieved by converting the emitted amounts of a greenhouse gas into an equivalent amount of carbon dioxide with the same global warming potential. Carbon dioxide equivalents are usually expressed as million metric tons of carbon dioxide equivalent, abbreviated MMTCDE.

The carbon dioxide equivalent for a gas is derived from the emitted tonnes of the greenhouse gas multiplied by the corresponding GWP:

MMTCDE = (million metric tons of a gas) * (GWP of the gas).

For example, the GWP for methane is 25 and for nitrous oxide 298. This means that emissions of 1 million metric tons of methane and nitrous oxide are equivalent to emissions of 25 and 298, respectively, million metric tons of carbon dioxide.



Οι άμεσες εκπομπές CO2 σχετίζονται με την εκτελούμενη γεωργική εργασία. Για τους υπολογισμούς των άμεσων εκπομπών CO2 λαμβάνεται υπόψη η συμβολή των καυσίμων, των λιπαντικών και της εργασίας.


CO2 emissions of fuel

Fuel consumption of agricultural machinery is calculated using ASABE standards and is related to available power and power required to successfully perform the job.

Calculated fuel quantities are converted to CO2 equivalents using the relevant conversion factors

CO2 emissions of oils

Oil consumption of agricultural machinery is calculated using ASABE standards and is related to the power required to successfully perform the work.

Calculated oil quantities are converted to CO2 equivalents using the relevant conversion factors.

Operational CO2 emissions

The manual work performed in the field is associated with a quantity of CO2 emissions that come from human breathing and the food they consume to sustain their lives. According to Hoolohan et al. (2013), an average diet was found to incorporate about 0.37 kg CO2eq per hour of the day, including both food consumed and food wasted. In addition, each person with their breathing produces about 0.04 kg of CO2eq per hour.


Detection / Terrain

Rapid and accurate detection of within-field variation is of major importance in precision agriculture as a means of supporting decision-making strategies to manage various challenges. The EM38-MK2 device (Gionics. Ltd., Mississauga, ON, Canada), a sensor that provides dense data sets, can be used to achieve the above goal. In particular, this instrument is designed for various agricultural applications, such as salt intrusion mapping, surface groundwater pollution monitoring, and surface soil thickness determination, and can cover large areas without the need for ground electrodes.

Advanced reading

By using the EM38-MK2, the measurement of both four-phase (conductivity) and in-phase (magnetic susceptibility) components can be provided at two different depth ranges and without any requirement for ground-device contact. The EM38-MK2 model is an electromagnetic induction device containing “transmitter” and “receiver” coils for producing a magnetic field and detecting an external magnetic field, respectively. There are two receiver coils, spaced 1 m and 0.5 m apart from the transmitter, providing data from true depth regions of 1.5 m and 0.75 m, respectively (when mounted in a vertical dipole orientation), and 0, 75 m and 0.375 m, respectively (when placed in horizontal dipole orientation) [1]. The single value of apparent electrical conductivity returned by the sensor is an integrated value based on the combination of depth-related instrument sensitivity and depth-dependent drivers of electrical conductivity.


Detection / Remote / Farming

EVI was developed to optimize the vegetation signal in areas with high biomass, i.e. high Leaf Area Index (LAI), where the Normalized Difference Vegetation Index (NDVI) may be saturated. In particular, similar to NDVI, EVI can be used to quantify the greenness of vegetation. However, EVI uses reflectance in the blue spectral region in order to correct for background ground signals and reduce atmospheric effects such as aerosol scattering.

Advanced reading

EVI uses values from the blue band (BLUE), an “L” value to adjust for the canopy background, a “G” gain factor, while the “C” values are incorporated as atmospheric drag factors. These estimates allow it to be calculated as a ratio between the RED and NIR values, also reducing saturation as well as background and ambient noise in most cases. The EVI can be calculated from:


where NIR and Red correspond to spectral reflectance measurements taken in the near-infrared and red regions, respectively. For example, the coefficients adopted in the MODIS-EVI algorithm are G = 2.5, C1 = 6, C2 = 7.5 and L=1.


Operations management

Field area capacity is defined as the time it takes a machine to perform a certain field area. It is directly related to machine speed, working width and field performance. Of course, the actual field area capacity differs from the theoretical one because the operating speed is not constant during a field job. At the same time, the yield of the field is dynamically affected by all the aforementioned factors.


Operations management

Field efficiency is a way of evaluating the performance of a field work (tillage, harvesting, etc.). It is a way of comparing the actual work done by a machine or tractor compared to the work that would be done if there was no loss of uptime.

Field efficiency is directly related to two main factors such as working width and speed. For a hypothetical case where a machine would operate at a constant width and speed without breaks or stops, the field efficiency would be close to 100%. This means that an agricultural machine can only achieve 100% field performance for a short period of time. The rest of the time the efficiency of the field is reduced, due to variations in the operating speed (for example in turns) or overlapping of the operating width due to possible obstacles. Overall, the main reason for the decline in field efficiency is lost time (non-productive time).

Advanced approach

Defining field efficiency in another way, we would say that it is the percentage of time that a tractor or a set of tractor-applicators is working efficiently in a field compared to the total time that the same machine is devoted to that operation[1]. Total uptime also includes idle time when a machine is not productive. This non-productive time can be spent in a number of ways.

  • Time spent in turns
  • Machine preparation in the field
  • Maintenance, breakdowns and any on-site repairs
  • Machine settings
  • Loading or unloading, in case there are work related to materials
  • Idle transport time within the field (e.g. for material replenishment)
  • Idle time during harvesting operations when the master unit waits for the service unit to return and resume operation

Based on the standards of agricultural machinery, typical values for the efficiency of agricultural operations can range from 50-90%. The variation in field efficiency is due to many factors, such as the theoretical capacity of the field area, the machine’s operating pattern, the machine’s flexibility, the skill and experience of the driver and, of course, the ground conditions.


The global warming potential (GWP) of a greenhouse gas describes its relative strength, taking into account each molecule of the gas and the length of time that gas remains active in the Earth’s atmosphere. As a prerequisite, GWP is currently calculated for the fixed time period (for all gases) of 100 years.

The reference gas for the various calculations is carbon dioxide (CO2) and for the 100-year period the GWP of CO2 is equal to 1.



Calculations of indirect CO2 emissions concern the evaluation of indirect emissions related to the agricultural work performed. Indirect CO2 emissions calculations take into account the contribution of embodied CO2 emissions from construction, repair and maintenance, housing and materials.


Construction CO2 emissions

Capital components have a certain amount of energy embedded in them due to their extraction, manufacture and maintenance, which can be calculated by multiplying the mass of each component by an appropriate CO2eq factor.

For tractors this average number is calculated as 4.25 kg CO2eq per kg machine. In the case of tools, the embodied CO2eq per kg of tool is considered as 10% of the corresponding embodied energy.

Repair and maintenance CO2 emissions

Energy and CO2eq for repairs and maintenance are usually expressed in the bibliography as a percentage of the total energy and CO2eq requirements for machinery production. For tractors this percentage is considered 45% and for tools 30%.

Housing CO2 emissions

The CO2eq for equipment housing and storage is related to the area covered by each machine or tractor. For machinery storage the carbon dioxide emissions correspond to 2.95 kgCO2eq/m2 of building or 15 kgCO2eq/ha of farm size. 15% more space than the coverage of the machine is taken into account during the calculation.

Material CO2 emissions

Materials used in agricultural operations that contribute to indirect CO2 emissions include fertilizers, propagation media (seeds) and agrochemicals. Fertilizers and agrochemicals are used to improve the quantity and quality of production. However, their excessive use has been linked to a multitude of adverse environmental effects. For example, excessive use of fertilizers contaminates groundwater with nitrates, making it unfit for human or animal consumption. In addition, fertilizer runoff into surface waters contributes significantly to eutrophication. The impact of pesticides and other agrochemicals is also significant, as during their application, non-target species, including humans, can be poisoned and affected. Some of the most critical environmental problems are associated with the use of certain types of relatively persistent agrochemicals.



Life cycle assessment (LCA) is an environmental impact assessment method that aggregates and evaluates all inputs, outputs and potential environmental impacts of a production system throughout its life cycle. In addition, it helps to identify the points where the most significant impacts occur, giving the user the possibility to develop strategies to improve the environmental performance of the product.

LCA has gained greater acceptance and is widely used by the majority of practitioners worldwide, and has also been standardized by ISO in the ISO 14040:2006 and ISO 14044:2006 standards.


Detection / Remote / Farming

NDVI is a widely used remote sensing index, usually analyzing measurements from satellites or drones, as a means of assessing whether or not a target contains live green vegetation. More specifically, NDVI is used in precision agriculture to help differentiate bare soil from grass, detect different crop stages, and assess crop health. The first reported use of NDVI was that of Rouse et al.. Since then, it has remained the most widely used index for detecting live green plant canopies when it comes to multispectral remote sensing data.

Advanced reading

Chlorophyll in plant leaves strongly absorbs visible light (from 400 to 700 nm) during the process of photosynthesis. In contrast, the cellular structure of leaves strongly reflects light in the near infrared (NIR) (from 700 to 1100 nm). Consequently, strong differences in plant reflectance can be exploited to define their spatial distribution.

Specifically, NDVI can be calculated from: NVDI= (NIR-RED)/(NIR+RED)

where NIR and RED correspond to spectral reflectance measurements taken in the near-infrared and near-red regions, respectively. In turn, NIR and Red are the same ratios of reflected to incoming radiation in each spectral band. Thus, their values range between 0 and 1, resulting in NDVI values between -1 and +1.


Detection / Remote / Terrain – Cultivation

The Soil-Adjusted Vegetation Index (SAVI) is a vegetation index used in situations where due to low plant cover the soil surface is exposed, such as in the early stages of crop development (less than 40% plant cover). It is used in the case of crop vegetation assessment instead of the Normalized Difference Vegetation Index (NDVI) as it minimizes the effect of soil moisture, soil color and type, among others, on the index values.

The figure below shows the NDVI and SAVI values for a wheat field in the early stages of development, where the NDVI rage is 0.54-0.84 while the SAVI rage: 0.77-1.20, with the second index providing more accurate field vegetation information compared to the first.